A tuneable component is a component that may exhibit, on demand, several levels of a given response. The variation in the response is adjusted by applying an electric field. Generally, the variation in the response is associated with a modification of the physico-chemical properties of at least one material within the structure of the component. Variation in a plurality of types of responses may thus be envisaged, for example a variation in electronic conductivity, in dielectric constant, in thermal conductivity, in refractive indices or in mechanical quantities.
Electrochemical actuation is one particular case: the variation in the response of the component is associated with a change in one or more properties of a material following an electrochemical reaction, which is obtained by applying an electric field.
Electrochemical actuation has a number of advantages in comparison to other (electrostatic, thermal, piezoelectric, etc.) actuating modes:
a very low energy consumption;
a low actuating voltage;
a very high adjustment precision, corresponding to a nanoscale theoretical variation in the electrochemical reactions.
Electrochemically actuatable tuneable components may be classed into two categories, depending on the mechanism of the electrochemical reaction at play.
First Category: Electrochemical Actuation by Insertion of Ions into an Electrode Material:
In this case, the insertion of an ion into a host structure of one material induces a change in the properties of said material and causes a variation in the response of the associated component. An example of this type of mechanism is reported in document Nature communications 5:4035-2014 and is illustrated in FIG. 1, which describes a variation in the thermal conductivity (in this case the variable property) of a material of LixCoO2 composition during the insertion of Li+ ions.
It will be noted that generally ion insertion is the mechanism that induces the change in the properties of the material. The ion insertion may take place via intercalation (in interstitial sites of the host structure), alloying, or even conversion mechanisms.
Second Category: Electrochemical Actuation by Filament Formation Through an Ionic Conductor:
In this case, the formation of a filament by dissolution of ions through an ionic conductor induces a change in the properties of the (ionic conductor+filament) assembly, in comparison to the ionic conductor alone. An example of this type of mechanism is reported in document Nano Letters 14-2401-2014 and is illustrated in FIG. 2, which describes a variation in the electronic conductivity between two electrodes of a metal/Al2O3/metal system caused by the formation and removal of a metal filament generated by Cu+ ions (producing a conductive state and an insulating state, respectively). It will be noted that in these cases the actuating and measuring electrodes are the same electrodes.
Thus, it will be understood that it is possible to modulate a substantial number of physico-chemical properties of materials, and to provide various component architectures that may be electrochemically actuated in the first or second of the modes described above.
Each of the actuating modes has a certain number of advantages and limitations, details of which will be given below in the present description.
Various approaches to tuneable electrochemical actuatable components have already been described in the patent literature.
Regarding Electrochemical Actuation by Insertion of Ions into an Electrode Material:
Patent application US20100003600 describes an electronic component of variable resistance employing the variation in the electronic conductivity of a material following insertion/deinsertion of Li+ ions. The insertion is implemented via the existence of a collector/positive electrode/electrolyte/negative electrode/collector stack deposited on a substrate, and the variation in the resistance is measured via coplanar interdigitated electrodes passing through the insertion electrode.
Patent application US20100075181 describes an electronic component of variable capacitance, using the increase and decrease in volume of a material during the insertion/deinsertion reactions. The volume change induces a variation in the distance between two metal plates and therefore a variation in the capacitance measurement across the terminals of the two plates in question. The insertion is ensured by the presence of a current collector/positive electrode/electrolyte/negative electrode/collector structure deposited on a substrate.
U.S. Pat. No. 7,262,899 describes a component having a variable optical response (waveguide structure). In this case a current collector/positive electrode/electrolyte/negative electrode/collector structure is present with a top current collector composed of a plurality of lines spaced apart by a given distance. The preceding structure allows lithium ions to be controllably inserted into one of the two electrodes and its refractive index to be modified. The modification of the indices leads as a result to a variation in the propagation of certain wavelengths and allows the filtering performance of the component to be modulated.
U.S. Pat. No. 7,652,907 describes an electronic component of variable response used as a memory for storing data. The component is a variable voltage source having a current collector/positive electrode/electrolyte/negative electrode/collector structure. The structure forms a battery that is characterized by a voltage profile during the insertion/deinsertion of ions into/from the electrodes; the profile in the context of this document consists of a plurality of plateaus, each voltage value of a plateau corresponds to a data code in the memory, 0 or 1 for example.
Regarding Electrochemical Actuation by Filament Formation Through an Ionic Conductor:
Patent application WO2014/025434 describes an electronic component, namely a resistive or conductive-bridge memory storage node (CBRAM for conductive-bridge random access memory).
The electronic component is characterized by a resistance that varies depending on whether or not a conductive filament is present through an ionic conductor (TaOx) placed between two metal contacts (Pd/Pd, see page 4 of the PDF document).
Patent EP2605274 describes a component of variable thermal conductivity. More specifically, an integrated circuit associated with a heat-removing system composed of a plurality of thermal bridges is described. Each thermal bridge is connected to a zone of the integrated circuit through a thermal switch composed of a metal/ionic conductor/metal trilayer.
The formation of a conductive filament through the ionic conductor allows passage from a (thermally) insulating mode to a conductive mode and allows the removal of the heat induced by the operation of the integrated circuit to be modulated.
U.S. Pat. No. 8,410,469 describes a radiofrequency switch (RF switch). The formation of a filament through an ionic conductor by application of a DC electric field between two electrodes allows the component to pass from an ON state to an OFF state and to ensure the conduction of a radiofrequency signal.
These two electrochemical actuating modes significantly influence the performance of the tuneable components in which they are implemented. Table 1 below summarises the major differences in their advantages and limitations. (Marked by (+) and (−), respectively).
TABLE 1Comparison of the performance of tuneable componentswith respect to the actuating mode implemented.ElectrochemicalElectrochemicalactuation byactuation byinsertionfilament formationGeometry2D/3D (+)1D (−)Modulation ratioAverage (−)High (+)IntermediateYes (+)No (−)statesResponse time>100 μs (−)<1 μs (+)Actuation voltagelow (1-2 V) (+)low (1-2 V) (+)Energylow (0.1-1 μJ) (+)low (0.1-1 μJ) (+)consumption
Both actuating modes are characterized by a low actuating voltage and a very low energy consumption, this being inherent to electrochemical actuation generally.
As regards the geometry of the component, electrochemical actuation by insertion has the advantage that the state variation may be planar or volumic (2D) whereas filamentary actuation is by construction one-dimensional.
This aspect is important insofar as the geometry may limit the number of possible component architectures: for example, it is not possible to form a filament of diameter covering a large area (a few tens or hundreds of μm2), or a substantial length (about one hundred microns), thereby limiting the size of a component to the aforementioned proportions.
As regards modulation ratio (ratio of the reachable extreme states of the variable quantity), actuation by filament formation is more advantageous. Specifically, the modulation of the response of a component is in this case associated with the intrinsic properties of two completely different materials, namely the ionic conductor (which is for example electronically insulating, thermally insulating and optically transmissive at a given wavelength) and the filament formed (which is electronically conductive, thermally conductive, and optically absorbent at a given wavelength, respectively), (other properties may be considered, the aforementioned are given by way of example).
In the case of actuation by insertion, the modulation is weaker insofar as the variation is achieved by changing the properties of a given material by passage from an inserted state to a deinserted state.
Electrochemical actuation by insertion allows a plurality of intermediate states of a given response to be obtained, each corresponding to a given degree of insertion of the ion in the structure of the host material.
The associated component is thus tuneable with a plurality of possible response levels in a given range.
In contrast, a component that is electrochemically actuated by filament formation only has two states, an ON state and an OFF state.
Response time is shorter in the case of a component that is electrochemically actuated by filament formation. Specifically, in this case only the mobility of ions within the ionic conductor plays a role and is the limiting step in the reaction mechanism.
In the case of insertion, in addition to the mobility in the ionic conductor, the diffusion of ions within the two (positive and negative) electrodes is also a factor, and thus the reaction is slower.